1. Field of the Invention
The present invention relates to a fuel cell potential measuring apparatus and a manufacturing method therefor, for measuring the electric potential of a fuel cell, in which an electrolyte electrode assembly, made up of an anode and a cathode provided on respective opposite sides of an electrolyte, is interposed between separators.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane (electrolyte) made up from a polymer ion exchange membrane. A membrane electrode assembly (MEA), which includes an anode and a cathode with an electrolyte membrane sandwiched between the anode and cathode, is interposed between separators. The membrane electrode assembly and the separators make up a unit of a fuel cell (unit cell) for generating electricity.
In this case, in the fuel cell, after the supply of a fuel gas and an oxygen-containing gas is halted accompanying stoppage of operation, it is still easy for reactions to occur between the residual fuel gas and oxygen-containing gas, which remain within the fuel cell. Owing thereto, in particular, there is a concern that the pressure inside the fuel gas flow passage will be lowered accompanying a reduction in volume of the fuel gas on the side of the fuel gas flow passage, and that the oxygen-containing gas will permeate through the electrolyte membrane from the side of the oxygen-containing gas flow passage and invade into the fuel gas flow passage. As a consequence thereof, inside the fuel gas flow passage, it is easy for a localized battery to be formed in regions where the fuel gas is improperly localized, and for a current to flow in a direction reverse to a current direction of normal power generation in regions where the oxygen-containing gas is improperly localized.
Consequently, an electrode potential measuring device for a fuel cell is known, as disclosed in Japanese Laid-Open Patent Publication No. 2004-095301. According to such a conventional technique, as shown in FIG. 7, an electrode potential measuring device 3 is incorporated in a fuel cell stack in which a pair of separators 2 sandwich a unit cell 1 therebetween. In the unit cell 1, a solid polymer electrolyte la is interposed between an oxygen electrode 1b and a fuel electrode 1c. 
The electrode potential measuring device 3 is equipped with a detector 4, a voltmeter 5, and conductive wires 6a, 6b. The detector 4 comprises a detection piece 4a disposed within the oxygen electrode 1b, a detection terminal 4b connected to a surface of the detection piece 4a, and a protective member 4c for protecting the detection terminal 4b. A portion of the oxygen electrode 1b is cut out, and the detection piece 4a is disposed therein in state of being insulated from the oxygen electrode 1b through an insulating member 4d. 
The detection piece 4a contacts the solid polymer electrolyte la and is ionically-conductive with the solid polymer electrolyte 1a. As a result thereof, abnormal potentials, occurring at locations where air (oxygen) is improperly localized on the side of the fuel electrode 1c, can be detected.
However, with the aforementioned conventional technique, a cutout portion is formed in the oxygen electrode 1b and the detection piece 4a is provided therein. Also, a gap between the detection piece 4a and the surrounding oxygen electrode 1b is filled with an insulating material serving as the insulating member 4d, so that the gap must be provided therebetween. Owing thereto, the structure becomes considerably complex. In addition, the influence of water, which is formed upon power generation, makes it difficult to accurately detect the potential.